1. Field of the Invention
This invention relates to and has among its objects the provision of novel methods for obtaining high resolution light scattering spectra such as those found in Raman spectroscopy. It is a particular object of the invention to secure these spectra free of artifacts due to the presence of optical components which distort the intensities of the scattered spectra. Further objects of the invention will be evident from the following description.
2. Description of the Prior Art
A spectrum is a display of the intensity of emitted light as a function of some varying characteristic, for example, wavelength, frequency, energy, etc. Spectroscopy is the study of the dependence of spectra on molecular structure and has proven to be a powerful investigative force in probing the mysteries of the natural universe in chemistry, physics, biology, and the like, and also in everyday uses in hospitals, crime detection bureaus, mines, factories, and so forth.
Spectroscopy is generally divided into sub-groups on the basis of wavelength or the type of excitation experienced by the molecule or atom irradiated. For example, Microwave spectroscopy studies rotational excitation of molecules; Infrared and Raman, vibrational and rotational changes; Ultraviolet and visible, electronic excitation accompanied by vibrational and rotational changes; and Nuclear magnetic resonance, excitation of magnetic atomic nuclei in a magnetic field induced by radio-frequency radiation.
A spectrometer, in the general sense, is an instrument that can produce a spectrum. Most spectrometers contain three elements: entrance and exit slits, a dispersing device to separate scattered light according to wavelength, and a suitable detection system to measure the light intensity as a function of wavelength. In Raman spectroscopy the sample to be studied is usually irradiated with a monochromatic high intensity light source (usually a laser) and the scattered light is imaged by a light collection lens on the entrance slit of a monochromator.
When molecules are irradiated, energy may be transmitted and absorbed as well as scattered. In addition, artifacts or anomalous light intensity may be present due to imperfections in optical components employed in the monochromator. These artifacts can obscure or interfere with the intensities of the scattered light spectrum, particularly in the portion of the spectrum near the wavelength of the laser.
The aformentioned artifacts can be avoided by first taking a spectrum from an irradiated sample, through an optical filter that removes intense Rayleigh scattering (at the same wavelength as the laser source), removing the sample, and then taking a spectrum from a white light source through the same optical filter. The two spectra can be ratioed to yield a spectrum from the irradiated sample free of artifacts from the optical filter and subsequent optics (monochromator, detector, etc.). This procedure works well if the measured scattered intensity varies slowly with wavelength. However, in cases of large intensity variations over a small wavelength range, e.g., high resolution spectrometry, the wavelength reproducibility of successive scans is not sufficiently good to ratio out artifacts. Reproducibility between scans must be of the order of 0.01 wavenumbers (cm.sup.-1) to obtain removal of artifacts, and known instruments do not possess this degree of reproducibility.